Iron's primary function, in both blood and muscle, is the transport of oxygen to cells. In blood, iron forms hemoglobin; in muscle, myoglobin. Iron is also a component of enzyme systems that are involved in the oxidation of glucose to produce energy.
Dietary iron is composed of two forms, nonheme and heme iron. Grains and vegetables contain nonheme iron; the iron from animal foods consists of approximately 2/5 heme iron and 3/5 nonheme iron. Heme iron is quickly absorbed into the small intestine, primarily in the duodenum. Because nonheme iron is ingested in the ferric form and reduced to ferrous iron, it is absorbed much more slowly. As iron enters the blood it is oxidized by ceruloplasmin, bound to transferrin, and carried to the liver and body tissues. Excess iron is stored in the form of ferritin.
Overall only about 5-10% of dietary iron is absorbed, though people who are iron deficient may absorb 10-20%. The amount of iron absorbed from a particular food can be dependent on other foods eaten at that time. Milk proteins, albumin, soy proteins, tea, and coffee all affect iron absorbtion. Phytate, a substance in wheat fiber and bran, rice, and nuts, also reduces absorption. The negative effect of such inhibitors can be reversed by eating meat or ascorbic acid.
Iron deficiency ,anemia, may result from inadequate dietary intake, excessive blood loss, or malabsorption due to iron inhibitors. Anemia is quite common in developed as well as developing countries. Anemia can result in weakness or tiredness, bleeding gums, nausea, tingling in limbs, confusion, and even dementia in extreme cases.
Iron overload may occur when the ability to excrete iron is hampered and excess iron is stored. This may be caused by a large increase in dietary intake, increased absorption, or increased breakdown of red blood cells. A genetic disorder, hemochromatosis, can result in iron overload and causing a bronze coloration of the skin, liver damage, or severe diabetes. Excessive blood transfusions can also result in high iron concentrations.
The assessment of iron status is complicated. There is no biochemical test that can reflect dietary intake. Instead, tests of body iron stores are used to assess iron status. The best test is of the serum ferritin level, since ferritin is the major storage form of iron. Adequacy of body stores can also be confirmed by serum iron and iron binding capacity tests. Hemoglobin is commonly used as a test of anemia and is a fair assessment of the total iron pool size in the body. Low serum iron, binding capacity, and hemoglobin may also be an indicator of chronic inflammation.
The RDA for iron is 10 mg/day for adult males, children, and postmenopausal women and 15 mg/day for women of childbearing age. During pregnancy the recommended intake is 30 mg, and for infants up to one year, 1 mg/kg/day.
The best sources of dietary iron are organ meats, followed by red meat and legumes. Green leafy vegetables, such as spinach, are also good sources.
Iron as a supplement is primarily used to treat iron deficiency and prevent anemia during pregnancy. The iron needs during pregnancy usually exceed what can be obtained through diet. A dose of 30 mg is usually taken in the form of iron succinate or iron fumarate. An excess of iron can result in the conditions previously listed.